This invention relates to a rotation detection apparatus for determining the degree of rotation of a rotational axis driven rotationally.
Actuators for rotating a rotational axis, and controllers for carrying out complicated controls such as a numerical control of rotation and a feed back control in response to rotational speed, have been proposed.
For example, when an arm of a robot for industrial use rotates to some predetermined angle, a predetermined operation such as welding or deburring is executed. Such rotation detection of a rotational axis is an indispensable technique today, when a lot of automatic controls are used and a lot of devices are prepared. Digital techniques of rotation detection are particularly prevalent since they are quite precise and the price of digital circuits is going down. In digital rotation detection, pulse signals in response to a rotation angle by a rotary encoder, etc. are received, and rotation of the rotational axis is determined by a count of the pulse signals.
Both an absolute type and an incremental type of encoder are known. Absolute encoders are capable of detecting rotation on the spot of switch-on, but they are expensive. Incremental encoders are cheap, but they are incapable of detecting rotation of the spot on switch-on, but rather require provision of an origin (zero point) by some apparatus. Once the origin is given degree of rotation can be detected by detecting relative rotation therefrom by integration.
Today, incremental encoders are more often used as rotation detectors because of their inexpensiveness, and when a lot of encoders are required incremental encoders are more often used.
Detection of the zero point of the rotational axis is indispensable to the use of an incremental type encoder as a rotation detector. One method for detection of the zero point is to detect a specific position of an output axis by a position detection sensor, and to determine the origin when an origin signal, which is output once every rotation of the incremental type encoder provided at an input axis connected to the output axis via a reduction gear, coincides with the position signal.
Such a rotation detector for the rotational axis, however, is not sufficient when the rotational axis rotates a lot, since the following problems remain. An absolute type rotation detector has a finite detection capacity and cannot detect a rotating degree of the rotational axis infinitely. Incremental type rotation detectors are much more often used when infinite rotations of the rotational axis are detected, but the following origin-determination is required. When the rotational axis rotates finitely, one point in that limited rotation is taken as the origin, but when the rotational axis rotates infinitely, precise origin determination cannot be obtained even if a special position on the output axis is taken as the origin.
Thus, an origin on the input axis does not necessarily coincide with that on the output exis even when the output axis is on the origin determination position since the rotation is detected at the input axis. In case of a reduction ratio 1:1, for example, to reassure the origin by rotating the output axis once or more, the position to be detected by the position detection sensor cannot be detected except when there is a specific relation (every ten rotations of the output axis), since with an output of once/rotation of the rotational axis, an output origin signal detected by the rotation detection sensor is generated per 1/1.1 rotation.
As can be seen in this case, when higher rotation is required, in order to set the origin in switching on or off, the output axis must be rotated until a simultaneous output of the origin signals of both the position detection sensor and the rotation detector is achieved.
However, in general, since the reduction ratio is large and a decimal part is long, considerable rotation is required to gain the origin determination position.
The pulse signals from the rotary encoder, etc. are continuously input as long as the rotational axis rotates. An overflow always occurs at some point, since the counting capacity is not infinite.
Thus, the rotation of the rotational axis was conventionally detected by setting the maximum in advance, for example, under such limitation as only one clockwise or counterclockwise rotation. This characteristic has affected the operation of the robot in its efficiency, since the mode of operation must have been limited. For example, when applying to the robot for industrial use, to rotate an arm counterclockwise for one operation and further counterclockwise for the next operation, the arm must be returned to the original position by rotating it clockwise after the first operation before proceeding to the next operation. It is one of the factors that generate a disadvantageous influence to decrease moving efficiency of the robot.